Ribozymes are catalytic active RNAs requiring a high magnesium(II) concentration to show folding and function in vitro [1,2]. In contrast, in vivo conditions are characterized by a highly crowded cellular environment and much lower ion concentration. Molecular crowding agents are used to mimic the cellular environment. However, particular physical/chemical properties explaining the co-solutes or the macromolecular crowders influence on folding and function of RNAs are poorly understood. In this study, we gain new insights on how polymer properties like dielectric constant, viscosity, diffusion and pore size influence the activity and folding of a large non-coding RNA, the group IIB intron ribozyme of S. cerevisiae [3,4]. We combined bulk activity assays, smFRET and NMR diffusion experiments screening the influence of volume fraction (%) and molecular weight (MW) of different crowding particles. We unveiled an optimal pore size of the crowder, i.e. the average distance of the crowding particles in solution, that matches the size of the ribozyme to maintain its catalytic activity even at lower magnesium(II) concentration.

Financial support from the European Research Council (MIRNA N° 259092 to R.K.O.S.), the Swiss National Science Foundation (to R.K.O.S.), and the Forschungskredit Grant of the University of Zurich (FK-14-096 and FK-15-095 to R.B.) are gratefully acknowledged.

[1] Richard Börner, Danny Kowerko, Helena Guisett-Miserach, Michelle F. Schaffer, Roland K.O. Sigel, Coord. Chem. Rev., 2016, 327-328, 123-142.
[2] Miriam Steiner, Krishanthi S. Karunatilaka, Roland K.O. Sigel, David Rueda, Proc. Natl. Acad. Sci. U.S.A., 2008, 105, 13853.
[3] Erica Fiorini, Richard Börner, Roland K.O. Sigel, Chimia, 2015, 69(4), 207. [4] Richard Börner, Erica Fiorini, Birgit Köhn, Michael Kovermann, Roland K.O. Sigel, 2017, in preparation.